Optical wiring boards, which are printed boards incorporating optical waveguides, are attracting attention as a means of resolving such persistent problems in various information processing equipment as high-frequency noise accompanying faster signals and inadequate transmission bandwidth.
In such optical wiring boards, mirror-reflecting films are formed on optical waveguides in order to bend light at desired angles, as for example when light is output from or, conversely, input to the board surface. As the method of forming such films, for example a method such as that of Non-patent Document 1 is known. This method comprises a process of forming a mirror-shape inclined face, and a process of forming a reflecting film on this surface. A summary of the method is explained as follows, referring to the figures.
In FIG. 1, (a) through (i) are schematic diagrams for explaining an example of a method of fabricating an optical wiring board, in which a mirror-reflecting film is formed on an optical waveguide, and show a representative method of fabricating a flexible-type optical wiring board.
First, a flexible electric board comprising a metal layer 1 of copper foil or similar and an insulating layer 2 formed from polyimide resin or similar is prepared ((a) of FIG. 1). On the face of the insulating layer 2 is formed a first cladding layer 3 ((b) of FIG. 1).
As the material of the cladding layer 3, resin having the desired transparency at the wavelength of the propagating light, for example 850 nm, is used. Various forms may be used, including liquid, semi-solidified film, or a UV-hardening material or thermosetting material. When a liquid is used, for example a spin coating method is applied, or in the case of a film, for example a vacuum lamination method is applied, and the liquid or the film is deposited on the insulating layer 2 and is hardened as necessary.
Next, a core layer 4 is formed ((c) and (d) of FIG. 1). The core layer 4 is a portion which confines light and enables propagation of light through total reflection at the interface with the cladding layer 3, and is normally patterned to a width of order several μm to several hundred μm.
As the constituent material of the core layer 4, a resin having a higher refractive index than the cladding layer 3, and having the desired transparency at the wavelength of the propagating light, for example 850 nm, is used. The form may be liquid or a semi-solidified film, and in general, a material which has UV hardening properties and can be patterned by UV lithography is employed.
The core layer 4 is generally formed by steps in which, after the entire surface is covered with film similarly to the cladding layer 3, masking is performed to mask unnecessary portions, and UV irradiation is then performed to harden only the necessary portions ((c) of FIG. 1), followed by a step in which the unnecessary portions are washed away (development) ((d) of FIG. 1).
Next, an inclined face 5 for mirror film formation is formed ((e) of FIG. 1). As the technique, for example a method employing a dicing blade, a method employing a router blade, or a method employing a laser may be used, in which normally the inclined face 5 is formed at an angle of approximately 45°. In the step immediately after formation of the inclined face 5, it may occur that the surface smoothness is insufficient for use as a mirror; in such cases, a varnish obtained by diluting the waveguide material may be applied to the machined face to improve the smoothness.
Next, the mirror-reflecting film 6 is formed on the inclined face 5 ((f) of FIG. 1). In forming the mirror-reflecting film 6, normally a vacuum process such as vacuum evaporation or sputtering is adopted. As the material of the mirror-reflecting film 6, of course a material with superior reflectivity in the wavelength region of the light to be transmitted is selected, but in consideration of reliability and cost, a material with a balance of properties is selected. For example, in Non-patent Document 1, Au (gold) is used.
Next, the core layer 4 in which the mirror-reflecting film 6 has been formed on the inclined face 5 is covered with a second cladding layer 7 ((g) of FIG. 1). In forming the second cladding layer 7, in general, the same material as in the first cladding layer 3 is adopted. The technique is also similar; in the case of a liquid, for example a spin-coating method is used, and in the case of a film form, for example a vacuum lamination method can be used for the formation (film deposition), followed by hardening performed as necessary.
After forming the second cladding layer 7, a coverlay layer 8 is formed thereupon to protect the light transmission layer ((h) of FIG. 1). As the material of the coverlay layer 8, a polyimide resin, polyester resin, or similar is used, and is applied by a vacuum press method or lamination method, followed by heating at approximately 140 to 170° C. to harden an adhesive layer.
Finally, processes such as formation of through-holes, circuit patterning, and solder resist, as well as metal plating are performed to complete the surface layer circuit 9 ((i) of FIG. 1). At this time, micrometer-order positioning precision may be required for the pads for mounting optical elements, and in such cases, a method of high-precision pad formation using laser machining is adopted, as for example described in Patent Document 1.    Patent Document 1: Japanese Patent Application Laid-open No. 2007-086210    Non-patent Document 1: Matsushita Electric Works Technical Report, Vol. 54, No. 3, “Optical/Electric Composite Flexible Print Board” (issued September 2006)